Kinetics of the Ag/KNO₃/CaCO₃ Catalyzed Aerobic Propylene Epoxidation and Effects of CO₂

Abstract

In this contribution we investigate the aerobic propylene epoxidation over a Ag/KNO 3 /CaCO 3 catalyst. The catalytic performance and surface speciation of the catalyst depends on the concentrations of feed modifiers such as ethyl chloride and nitric oxide, added to the feed to improve the selectivity by suppressing total combustion to CO 2 . After a kinetic character-ization of the system, we investigate the kinetics in the presence of CO 2 . We demonstrate that CO 2 alters the rate dependence in both reactants, affects the activation of O 2 , and results in the same maximum rate of propylene oxide formation.[a] Dr.

Critical Role of Chlorinated Hydrocarbons in Propylene Epoxidation over K-Ag/CaCO3

“…K-promoted Ag/CaCO 3 was synthesized utilizing methods previously reported in the literature. ,,, Oxalic acid dihydrate (46.7 mmol; Sigma, 99%) was first stirred in a beaker containing a 1:1 (wt.) solution of ethylenediamine and deionized (DI) water (4.21 g each), forming a white liquid solution.…”

“…Steady-state conversions of propylene ( X ), oxygen ( X O2 ), and ethyl chloride ( X C2H5Cl ) were calculated utilizing eqs – where n C,j and n O,j represent the number of carbon and oxygen atoms, respectively, in each product j. As CO 2 is a well-known epoxidation promoter, , it was co-fed at high concentrations (∼10 vol %) within all reactions, necessitating H 2 O flow rates to be measured to quantify combustion site-time yields via the stoichiometry of propylene combustion assuming no H 2 generation. Propylene conversions were <3%, O 2 conversions were <10%, and C 2 H 5 Cl conversions were <10% in all reactions reported.…”

“…Steady-state conversions of propylene (X), oxygen (X O2 ), and ethyl chloride (X C2H5Cl ) were calculated utilizing eqs 5−7 where n C,j and n O,j represent the number of carbon and oxygen atoms, respectively, in each product j. As CO 2 is a well-known epoxidation promoter, 8,9 3 NTP s −1 He) without reducing temperature or removing the catalyst bed from the reactor. No loss of Cl* is expected during this period, as thermal desorption of Cl* from Ag has been found to occur at temperatures >690 K under vacuum 29 �significantly higher than our reaction temperature of 513 K. Utilizing the batch configuration described in Section 2.2., the catalyst bed was then titrated in a recirculating mixture of 9% C 2 H 6 and 3% O 2 , 150 ppm NO, and 0.3% CH 4 (V sys ∼ 100 or 1050 cm 3 , P sys ∼ 110 or 150 kPa, T ∼ 513 K) during which C 2 H 6 reacts with surface Cl* to stoichiometrically evolve C 2 H 5 Cl (eq 2) which was then normalized by bed-total Ag surface sites to yield Cl* coverages (denoted "θ Cl ").…”

“…Direct oxidation of C 3 H 6 to propylene oxide (PO) by O 2 environmentally and economically preferable to current industrial processes for synthesis of PO − achieves a maximal reported selectivity of only ∼50–60% over Ag/CaCO 3 catalysts in the presence of a variety of gas-phase and inorganic promoters including continuously co-fed trace alkyl chlorides (∼20–150 Pa C 2 H 5 Cl). − Alkyl chlorides, albeit in far lower partial pressure (e.g., ∼0.5-3 Pa C 2 H 5 Cl), are well-known gas-phase promoters for Ag-catalyzed epoxidation of other olefins by O 2 , most prominently ethylene and 1,3-butadieneeach achieving ≥90% epoxide selectivity over Ag/α-Al 2 O 3 in the presence of co-fed alkyl chlorides as well as inorganic promoters such as Cs, Mo, and Re. − Within ethylene epoxidation, these trace chlorinated hydrocarbons are known to slowly deposit Cl adatoms on Ag surfaces (denoted “Cl*”; eq ) which are also known to be removed under epoxidation conditions via oxychlorination (eq ) of trace, co-present hydrocarbons (e.g., ∼0.5 vol % C 2 H 6 ). ,− Dehydrochlorination : normalC x normalH y Cl + 0.25em * prefix+ normalO * prefix→ normalC x normalH y − 1 / normalO * prefix+ Cl * Oxychlorination : normalC x normalH y + normalO * prefix+ Cl * prefix→ normalC x normalH ...…”

Ag supported on alumina for the epoxidation of 1-hexene with molecular oxygen: the effect of Ag+/Ag0

“…). 7,8 Nevertheless, the tedious catalyst preparation, product separation and complex protocol hinder their application, whereas heterogeneous catalysts, with the merits of easy product separation, 9,10 mild reaction conditions, and stability have gradually drawn more and more attention. 11 The direct epoxidation of linear terminal alkenes using O 2 is a “dream reaction” in heterogeneous catalysis, but the selective oxidation to epoxides is rather challenging.…”